This invention pertains to fuel cells and, in particular, to fuel cells in which there is internal reforming of the hydrocarbon content of the fuel cell supply gas.
It is known in the design of fuel cells, such as, for example, molten carbonate and phosphoric acid cells, to internally reform the hydrocarbon content of the fuel supply gas. Such hydrocarbon content usually contains methane which itself is relatively electrochemically inactive, but which when reformed produces hydrogen and carbon monoxide which are significantly more electrochemically active and, therefore, can readily participate in the fuel cell reaction. Reforming internal of the fuel cell is beneficial in that the reforming reaction is endothermic and serves to offset heat generated in the cell during operation. Accordingly, by internal reforming, the load on the fuel cell cooling system can be reduced.
U.S. Pat. No. 3,488,226 discloses an internal reforming scheme in which the reforming catalyst is situated within the anode electrode gas chamber. The hydrocarbon content of the supply fuel gas is thus substantially all reformed during its passage through the anode chamber, and, moreover, is immediately available to the cell anode upon reformation. A drawback of this arrangement, however, is that the endothermic nature of the reforming reaction results in cold spots in the anode chamber which cause condensation of electrolyte vapor transmitted to the anode chamber through the gas-diffusion anode electrode. Such condensation, in turn, may severely reduce catalytic activity and, as a result, the reforming reaction. A second drawback of this arrangement is that because the electrolyte is a liquid at operating temperature a film of electrolyte will creep over the anode chamber surfaces and contaminate the catalyst. Such film creepage, in turn, may severely reduce the catalytic activity and, as a result, the reforming reaction.
U.S. Pat. No. 4,182,895, assigned to the same assignee hereof, attempts to avoid electrolyte vapor condensation and electrolyte film creepage by providing an electrolyte-isolated chamber in which the catalyst is placed and in which the reforming reaction takes place. Fuel supply gas reformed in the electrolyte-isolated chamber is then introduced into the anode (electrolyte-communicative) chamber for electrochemical reaction. Owing to the isolation of the reforming chamber from the electrolyte, electrolyte vapor condensation and electrolyte film creepage on the reforming catalyst does not occur and catalyst activity is preserved.
In the '795 patent arrangement, however, reformed gas is not immediately available to the anode chamber, but must be introduced into such chamber subsequent to reformation. Furthermore, since the product gas containing water (H.sub.2 O) is not available to the electrolyte isolated channel, reformation is not driven to completion, i.e., reformation is limited to thermodynamic equilibrum which is about 80% of total conversion.
U.S. Pat. No. 4,365,007, also assigned to the same assignee hereof, discloses a fuel cell internal reforming arrangement which is designed to provide immediate availability of reformed gas without deactivation of the reforming catalyst by the electrolyte. In this arrangement, a gas porous member is used to define first and second fuel cell passages. The first passage is in communication with the fuel cell anode electrode and with the second passage which includes the reforming catalyst. The arrangement is further provided with a differential pressure means which causes flow of gas reformed in the second passage to the first passage and retards electrolyte vapor from flowing from the first passage to the second passage. As a result, poisoning of the reforming catalyst by vapor condensation is prevented.
The gas porous member in the '007 patent system is in the form of a corrugated plate having perforations over its entire surface and along the length of the first and second passages. This patent also states that increased uniformity of the gas flow distribution can be realized by using an increased number of performations at the gas exit relative to the gas entrance end. While this arrangement thus provides immediate availability of the reformed gas as well as catalyst protection, it also requires a differential pressure means to be utilized. Also, like the arrangement of the '895 patent, reformation is not driven to completion because of the unavailability of the product gases of the electrochemical reaction to the second passage.
It is an object of the present invention to provide a fuel cell having improved internal reforming.
It is a further object of the present invention to provide a simplified fuel cell arrangement in which internal reforming is carried out such that the reformed gas is made substantially immediately available to the cell electrode, while at the same time avoiding electrolyte contamination by vapor condensation and film creepage on the reforming catalyst and enabling reformation to go to substantial completion.